1. Field of the Invention
The invention relates to a ceramic substrate, and more particularly to a ceramic main body with a planar buffer layer.
2. Description of the Related Art
Ceramic materials are widely used in communication devices, computers, medical devices, and military devices, due to their high mechanical strength, thermal conductivity, thermostability, and superior dielectric characteristics. Ceramic substrates are increasingly being applied as the carriers of micro electro mechanical systems, solar cells, and light emitting diodes for applications in semiconductor, storage device, display, and optoelectronic product.
One major disadvantage of conventional ceramic substrates is the extremely rough surfaces, there are a plurality of various sized pores exist on the surfaces, particularly, when forming semiconductor elements or depositing specific layers, such as Si, Ge, SiGe, CIGS (Cu—In—Ga—Se system with different molar radio), GaN, GaAs, ZnO, ITO, GZO (Ga doped ZnO) or AZO (Al doped ZnO) on a conventional ceramic substrate, the high roughness average (Ra) surface thereof decreases fabrication yields. The Ra has be defined the mathematics formula as
      Ra    =                                                      (                                                h                  ⁢                                                                          ⁢                  1                                +                                  h                  ⁢                                                                          ⁢                  2                                +                                  h                  ⁢                                                                          ⁢                  3                                +                                  …                  ⁢                                                                          ⁢                  hn                                            )                        ·            Δ                    ⁢                                          ⁢          x                L            =                        ∑                                    f              ⁡                              (                x                )                                      ⁢                          ⅆ              x                                      L              ,where the L is the measurement length and segment to n section with the same length x, h1 is the height at position 1, h2 is the height at position 2 and deduced by analogy to hn.
Several methods have been employed to solve the roughness average problem, which include mechanical polishing process, chemical mechanical planarization (CMP) process, chemical etching process, spin-on glass process, and BPSG (borophosphosilicate glass) reflow treatment process.
In practice, the roughness average of a rigid substrate can be reduced by mechanical polishing. Of the available industry methods, chemical mechanical planarization (CMP) is complex, costly and time consuming. Due to the characteristics of the ceramic substrate, pores are produced during mechanical polishing. Further, pollutants may be introduced into the ceramic substrate during the CMP process. As for the BPSG reflow treatment process, it is a silane-based treatment process with PH3 and B2H6 dopant sources, however, PH3 and B2H6 are toxic gases. Additionally, the BPSG reflow treatment process is applied exclusively for isolation the ceramic substrate before a metallization process. Meanwhile, the spin-on glass process provides only partial planarization of the ceramic substrate surface and tends to cause outgassing. Furthermore, the adhesion of spin-on glass layers onto ceramic substrates is poor.
Accordingly, the disadvantages of conventional methods for improving roughness average of ceramic substrates are high cost and complex process steps. Further, conventional ceramic substrates cannot be used in high temperature processes due to the coefficient of thermal expansion mismatch with device or further deposited film thereof, which resulting in limited applicability.
In order to solve the aforementioned problems, vitreous materials are often used as a coating for some conventional ceramic substrates to improve surface characteristics thereof, and further enhance applicability. For example, conventional ceramic substrates can be fabricated to have higher chemical resistance, low liquid or gas permeation resistance, smoother surfaces, higher mechanical strength, higher friction resistance, and higher abrasion resistance, etc, thus enhancing its applicability and value of use. Since high temperature processing is required for good electronic devices performance, a matched or relatively lower coefficient of thermal expansions for conventional ceramic substrates is desired.
For example, in optoelectronic devices employing a conventional ceramic substrate deteriorate the performance. Specifically, the different coefficients of thermal expansion between the optoelectronic material (such as glass or quartz) and the conventional ceramic substrate during high temperature process will deformation or distortion due to the thermal stress therein. Further, in light emitting diodes or solar cells employing conventional ceramic materials as carrier substrates, the film or device on the ceramic substrates may warp or swell due to the large differences in the coefficients of thermal expansion of both films and substrates. Further, the high roughness average may increase the failure rates of the devices.
Therefore, it is necessary to develop a ceramic substrate with low roughness average, suitable coefficient of thermal expansion and higher heat resistant ability to solve the previously described problems.